(defun sum (n)
  "Sum `n'."
  (let ((s 0))
    (dotimes (i n s)
      (incf s i))))

(sum 10)

(defun addn (n)
  "Function to add `n' to its argument."
  #'(lambda (x) (+ n x)))

(funcall (addn 4) 1)

(eql () nil)

(cons 'a '(b c d))

(caddr '(a b c d))

(listp nil)

(null nil)

(if nil
    :t
    :nil)

(caddr nil)
(third nil)
